Combine stock and water in a large pot over high heat. Bring to a boil, then reduce heat to a simmer.
While broth is warming, heat oil over low heat in a large pot. Sauté onions until starting to become translucent.
Add pumpkin to onions, stirring to incorporate. Let cook, stirring occasionally and adding broth as needed to prevent burning.
Add rice, stirring to coat well with the butter, and toast briefly.
Increase heat to medium, add wine and 1 cup of the broth, and stir to incorporate.
Continue cooking, stirring occasionally and adding additional stock once the previous amount has evaporated until risotto is al dente.
Remove from heat, adjust salt, and add additional broth to thin if desired.
Stir butter and parmesan in to risotto to thoroughly incorporate. Let reset for a few minutes before serving.